Critically ill premature infants' adaptation to environmental stressors is dependent upon the balance between the parasympathetic and sympathetic branches of the autonomic nervous system. This balance can be quantified via analysis of heart period using advanced statistical methods including power spectral analysis and growth curve analysis. Current digital technology and advances in statistical methods makes it possible to bring scientific rigor to real time clinical decision-making regarding autonomic nervous system balance and infant responses to environmental stressors in the neonatal critical care unit. The focus of this study is to make the first steps in this area by analyzing heart period data obtained from critically ill, intubated very-low-birth-weight (I-VLBW) infants with respiratory distress syndrome or bronchopulmonary dysplasia. Sophisticated statistical techniques, including power spectral analysis and growth curve analysis, will be employed. The specific aims of this study are to (1) identify characteristic heart period power in the low-frequency and high-frequency regions for premature I-VLBW infants (power in this context is the amount of influence exerted on the low- and high-frequency regions of heart period); and (2) describe and categorize patterns of intra-individual variability and change of heart period power in the low- and high-frequency regions over time, during sleep and wake periods, and during nursing disruptions. The proposed research is a secondary analysis of previously collected data and is in response to RFA HL-01-016, NHLBI Innovative Research Grant Program. The parent study for this investigation was a clinical trial that tested the intervention of maternal skin-to-skin holding on physiological responses of I-VLBW infants. An experimental interrupted time-series crossover design was employed. Infants were randomly assigned to treatment order and served as their own controls. Heart period data were acquired in multiple 5-minute epochs during the study period. The long term goals of this proposed study are to (1) study heart period in a larger sample of critically ill infants to determine normative values of heart period power, (2) test the impact of developmental care on heart period power, and (3) develop and test interventions that would promote balance in the low- and high-frequency regions of heart period power